1. Field of the Invention
The present invention relates to a process for continuously producing shaped articles of polystyrene foam, and more particularly to a process for continuously producing shaped articles of polystyrene foam which have a small wall thickness, can be stacked to a small height, have sufficient strength for use and are economical.
2. Description of the Prior Art
In a process for producing shaped articles of polystyrene foam, it is known to use an extruder. This process comprises the steps of extruding a polystyrene resin composition along with a blowing agent by an extruder to obtain a foamed sheet, aging the sheet by allowing the sheet to stand usually for 2 to 3 weeks as wound up on a roll, heating the sheet to a temperature not lower than the softening point of the sheet and forming the sheet with molds. With this process, the extruding step and the forming step are entirely independent and there is the need to age the foamed sheet for a prolonged period of time between the two steps. The process is therefore unfavorable for a continuous operation and also involves problems economically. When the aging step is not performed, there arise such problems that the sheet is not shapable properly and the product undergoes dimensional variations with the lapse of time.
To overcome these problems, it has been proposed to supply an extruded sheet to a rotary vacuum mold for continuously producing shaped articles of thermoplastic resin foam (Japanese patent publication No. 37134/1973). However, this process, in which the heat of extrusion is utilized for forming, has problems in that the sheet is not fully stretchable and is not amenable to deep drawing, the shaped product obtained is low in strength, extreme difficulties are encountered in temperature control, and it is difficult to keep the speed of forming. Furthermore, the process is not applicable to sheets of large width and fails to achieve a high production efficiency due to great losses.
Accordingly it has been desired to provide a continuous production process free of the foregoing problems.
On the other hand, trays, partitioned sheets and like shaped articles of polystyrene foam useful for retailing foods including vegetables, fruits, meat, salad, etc. usually have an expansion ratio of at least 10 and a wall thickness of at least 2 mm and are tough but process the drawbacks of having a large stack height (the height of 100 trays stacked, the same as hereinafter) and necessitating a prolonged forming cycle (the period of time required for feeding a sheet to the molds and completely forming the sheet into the desired shape). Although trays of unfoamed polystyrene resin are available which have a wall thickness of about 0.3 mm and a small stack height, such a product requires a longer forming cycle and a larger amount of resin and is therefore uneconomical.
Accordingly it has been desired to provide of polystyrene foamed articles which have a small wall thickness, can be produced by a short forming cycle and are economical.